Modern steering systems also comprise electronic power assisted steering in addition to the conventional servo powered assistance for the driver. The electronic power assisted steering renders possible, for example, the superimposition of steering torques within the scope of assistance systems for the driver, which increases comfort and safety. In addition, the electronic power assisted steering renders it possible to reduce the disturbing influences of torque that are transmitted to the steering wheel from the vehicle or from the road surface. The servo powered assistance is defined in this connection as a torque superimposition system that is dependent upon the driver manual torque and increases the driver manual torque. The electronically powered assistance is generally independent from the driver manual torque and can even counteract the driver manual torque in order to provide power assisted steering torque, correction steering torque or assistance steering torque, by way of example in order to prevent any departure from the driving lane or in order to provide the driver with information with respect to the steering situation.
With the introduction of electronic power assisted steering systems for road-borne vehicles, it is not only necessary to demonstrate the mechanical integrity of the steering system but it is also necessary to ensure that the driver retains control of the vehicle at all times and that protection is provided against any possible electrical failures. With the large number of assist function and driver assistance functions, electrically or digitally transmitted signals are used ever more frequently in order to calculate the steering torque for the electronic power assisted steering. Furthermore, these steering systems provide the opportunity to suppress recognized disturbance variables by means of a correction steering torque or even to influence the vehicle trajectory when recognizing a situation using environment sensors. In the case of the corresponding sensor system, or rather the electrical or digital processing and transmission of the signals, it is necessary to also take into consideration the possibility that an incorrect superimposition of an assisting steering torque, correction steering torque or assistance steering torque cannot be fully suppressed in certain circumstances. It is therefore necessary to limit the effect of potential erroneous steering torques on the reaction of the vehicle in order to ensure that the driver has full control of his vehicle at all times.
The steering wheel is the haptic interface to the driver. If dangerous situations are to be reliably avoided during the superimposition of the steering torques by virtue of the electronically powered assistance, it is important that a superimposed steering torque is always of such a magnitude that the driver remains in control of the vehicle. This also applies in particular in the event of an error, in other words in the case of a superimposed torque that is not justified by a driving situation. The electronically powered assistance is therefore to be designed in such a manner that the worst possible steering torque superimposition in the case of an error does not pose a danger for the entire system comprising driver, vehicle and vehicle situation.
Known approaches for avoiding dangers of this type reside in limiting the amplitude and dynamics of the steering torque of the electronically powered assistance. Customer clinics or rather expert clinics are conducted in order to ascertain controllable thresholds. Owing to varying evaluations, it is necessary to assume the worst case scenario, in other words, any limitation in the case of an error must be geared towards the most critical case or rather to the most critical evaluation. As a result, the possible performance of an electronically powered assistance cannot be fully exploited, i.e., it is not always possible to superimpose steering torques of this type on the driver manual torque, as would be expedient or even necessary for the respective driving situation.
The object of the present disclosure is therefore to provide a method and an apparatus for limiting an assisting steering torque in a steering system having electronic powered assisted steering, which method can more fully exploit the possible performance of the electronically powered assistance.